Counter-rotating open-rotor (cror)

ABSTRACT

A method of controlling a Counter-Rotating Open-Rotor (CROR) includes measuring a speed of a first rotor and deriving a speed of a second rotor and controlling a pitch of the first rotor and the second rotor.

The present disclosure claims priority to U.S. Provisional PatentApplication No. 61/345,725, filed May 18, 2010 and U.S. ProvisionalPatent Application No. 61/345,743, filed May 18, 2010.

BACKGROUND

The present disclosure relates to gas turbine engines, and moreparticularly to Beta operation of a Counter-Rotating Open-Rotor (CROR).

A Counter-Rotating Open-Rotor (CROR) includes a gas turbine engine withcounter-rotating un-ducted rotors outside a nacelle structure. Propfansare also known as ultra-high bypass (UHB) engines and, most recently,open rotor jet engines. The design is intended to offer the speed andperformance of a turbofan, with the fuel economy of a turboprop.

CRORs may have particular challenges in terms of aerodynamics,aeroacoustics and structural dynamics as the forward and aft rotors areoutside the nacelle structure and are positioned relatively closetogether which may result in rotor/rotor interactions.

SUMMARY

A method of controlling a Counter-Rotating Open-Rotor (CROR) accordingto an exemplary aspect of the present disclosure includes measuring aspeed of a first rotor and deriving a speed of a second rotor andcontrolling a pitch of the first rotor and the second rotor.

A method of controlling a Counter-Rotating Open-Rotor (CROR) accordingto an exemplary aspect of the present disclosure includes entering BetaControl and preventing one rotor of a first rotor and a second rotorfrom entering a CP (coefficient of power) bucket.

A Counter-Rotating Open-Rotor (CROR) according to an exemplary aspect ofthe present disclosure includes a rotor control module operable toderive a speed of one of a first rotor and a second rotor and measure aspeed of the other one of said first rotor and said second rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features will become apparent to those skilled in the art fromthe following detailed description of the disclosed non-limitingembodiment. The drawings that accompany the detailed description can bebriefly described as follows:

FIG. 1 is a general perspective view an exemplary gas turbine engineembodiment for use with a Counter-Rotating Open-Rotor (CROR) propfan;

FIG. 2 is an expanded view of the CROR propfan;

FIG. 3 is a schematic representation of a rotor control positionschedule;

FIG. 4 is an expanded schematic view of a transfer bearing for the CRORpropfan;

FIG. 5 is a block diagram of one non-limiting embodiment of a controlsystem to control the counter-rotating un-ducted rotors of the CRORpropfan;

FIG. 6 is a schematic representation of a control map of a forward rotorof the CROR propfan of FIG. 5;

FIG. 7 is a schematic representation of a control map of an aft rotor ofthe CROR propfan of FIG. 5;

FIG. 8 is a schematic representation of a feedback system for an aftrotor of the CROR propfan of FIG. 5;

FIG. 9 is a block diagram of another non-limiting embodiment of acontrol system to control the counter-rotating un-ducted rotors of theCROR propfan;

FIG. 10 is a schematic representation of a control map of a forwardrotor of the CROR propfan of FIG. 9;

FIG. 11 is a schematic representation of a control map of an aft rotorof the CROR propfan of FIG. 9;

FIG. 12 is a schematic representation of a counterweight system of theCROR propfan of FIG. 9;

FIG. 13 is a block diagram of another non-limiting embodiment of acontrol system to control the counter-rotating un-ducted rotors of theCROR propfan;

FIG. 14 is a schematic representation of a control map of a forwardrotor of the CROR propfan of FIG. 13; and

FIG. 15 is a schematic representation of a control map of an aft rotorof the CROR propfan of FIG. 13.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a Counter-Rotating Open-Rotor (CROR)20. The CROR 20 generally includes a gas turbine engine 22 withcounter-rotating un-ducted rotors 24, 26 outside of a nacelle structure28 on a central longitudinal engine axis A. The CROR 20 may beconfigured as a tractor (rotors ahead of the engine in a pullingconfiguration), or as a pusher (shown). Although depicted as aparticular architecture in the disclosed non-limiting embodiment, itshould be understood that the concepts described herein are applicableto other architectures.

The gas turbine engine 22 generally incorporates a compressor section30, a combustor section 32 and a turbine section 34 with a power turbine36. The power turbine 36 provides a speed and torque output to drive agear system 38 which drives the counter-rotating un-ducted rotors 24,26. The sections are defined along the central longitudinal engine axisA and the gear system 38 may be located axially between thecounter-rotating un-ducted rotors 24, 26.

With reference to FIG. 2, the gear system 38 in the disclosednon-limiting embodiment is a planetary, differential gearbox whichgenerally includes a sun gear 40 driven by the power turbine 36, amultiple of planet gears 42, a planet carrier 44, and a ring gear 46which rotate relative to a fixed structure 48. The forward rotor 24rotates with the planet carrier 44 and the aft rotor 26 counter rotateswith the ring gear 46.

The counter-rotating un-ducted rotors 24, 26 each includes a multiple ofpropeller blades 24B, 26B (one shown) which are connected with therespective planet carrier 44 and ring gear 46 through a pitch changesystem 50, 52. The pitch change systems 50, 52 include an axiallymovable forward pitch change actuator 54 and axially movable aft pitchchange actuator 56 to pitch the rotor blades 24B, 26B about a respectiverotor blade axis B1, B2 to achieve the desired propeller mode such asFeather, Forward Speed Governing, CP (coefficient of power) Bucket “keepout zone”, and Reverse (FIG. 3).

The pitch change systems 50, 52 may include linear hydraulic actuationsystems with metered pressures that may be ducted to an oil transfertube 60 which contains at least four separate hydraulic passages (FIG.4). Oil supplied through the oil transfer tube 60 to the pitch changeactuators 54, 56 may flow through a four-land transfer bearing 62located at the aft end of the oil transfer tube 60. The transfer bearing62 provides the hydraulic connection between the stationary and rotatinghardware. Two pressures (coarse and fine pitch—forward rotor) from theoil transfer tube 60 are provided to the forward pitch change actuator54, while the other two pressures (coarse and fine pitch—aft rotor) areprovided to the aft pitch change actuator 56 through an aft transferbearing 64.

Each of the pitch change actuators 54, 56 includes a dual-acting pistonwith differential areas sized in accordance with pitch change actuatorstructural and performance requirements. Each pitch change actuator 54,56 includes a pitch change yoke 54Y, 56Y which transmits the linearforce output of the pitch change actuator 54, 56 to a trunnion 24T, 26Tat the base of each rotor blade 24B, 26B.

With reference to FIG. 3, a rotor control module 58 provides meteringand control of oil supplied to the pitch change system 50, 52 to changethe pitch of the rotors 24, 26 of the CROR 20. It should be understoodthat FIG. 3 is a block diagram representation of functions that may beenacted in either dedicated hardware circuitry or programmed softwareroutines capable of execution in a microprocessor based electroniccontrol environment such as rotor control module 58. The rotor controlmodule 58 uses two primary propeller control modes for a constant speedpropeller system: Fixed Speed Control and Beta Control. In flight athigh power, the CROR 20 is in fixed speed control mode which, intechnical parlance, operates as an isochronous governor. That is, therotors 24, 26 are essentially the governor for the gas turbine enginepower turbine 36. So the rotor control module 58 sets a fixed speedrequirement and then adjusts rotor blade angle to absorb whatever powerthe gas turbine engine 22 outputs that the rotor blade speed will remainfixed. Although, there may be additional selectable rotor speeds forparticular flight conditions, once that speed is selected, power changeis accomplished through pitch change of the rotor blades rather thanspeed change to assure rotor blade frequencies are maintained inpredesigned regions and optimal performance is available.

As power is decreased, the effectiveness of the rotors 24, 26 as thegovernor for the power turbine 36 becomes less. That is, slop increasesas pitch approaches the CP bucket where relatively large changes inrotor blade angle do not result in much change in power absorption. TheCP bucket “keep out zone” is the region where the rotors 24, 26 cannotbe effectively controlled through a change in rotor blade pitch. Forexample, if a rotor blade pitch lower than the low pitch stop iscommanded in flight, the forward airspeed of the aircraft may windmillthe rotor which increases power to the system such that the rotor mayoverspeed. Movement of blade angle further into the bucket in thedecrease pitch direction will result in increasing overspeed until bladeangle has reached the reverse region where further decreases in bladeangle will begin to absorb power thus reducing propeller speed—which iscounter-intuitive. Under certain conditions, this CP bucket “keep outzone” may result in reversed commands which overspeed the rotors.

The low pitch stop is the lowest blade pitch angle in the fixed speedcontrol mode below which the rotor speed can not be effectivelycontrolled. The low pitch stop, however, must be transited through toenter reverse pitch. The rotor control module 58 switches to the BetaControl mode where rotor blade pitch is directly commanded rather thanspeed. Beta Control mode is typically used after aircraft touch down andwhen the power is relatively low such as ground idle, ground operations,or reverse operations.

With reference to FIG. 5, one non-limiting embodiment of a controlsystem 80 with inputs and outputs to control the counter-rotatingun-ducted rotors 24, 26 of the CROR 20. Mechanical connections aredepicted as the heavy lines from the gas turbine engine 22 to the gearsystem 38 then split to the counter-rotating un-ducted rotors 24, 26.The heavy black line function boxes between the mechanical pathsrepresent the mathematical relationships due to the mechanical systems.

The rotor control module 58 communicates with the pitch change system50, 52 and an engine control module 82 such as a Full Authority DigitalElectronic Control (FADEC) that communicates with the gas turbine engine22. The control modules 58, 82 execute algorithms that are disclosed interms of functional blocks and it should be understood by those skilledin the art with the benefit of this disclosure that these functions maybe enacted in either dedicated hardware circuitry or programmed softwareroutines capable of execution in microprocessor based electronicscontrol module embodiments of various configurations.

From the gas turbine engine 22, Npt is the speed of the power turbine 36and T is the torque of the power turbine 36 which is essentially thepower output into the gear system 38. Output from the gear system 38 istwo paths because the planetary differential gearbox provides the twocounter-rotating outputs for the counter-rotating un-ducted rotors 24,26.

T1 and Nr1 are torque and speed to the forward rotor 24 and T2 and Nr2are torque and speed to the aft rotor 26. The physics of the gear system38 provide the following mathematical relationships:

Nr1+Nr2=C*Npt  (1)

T1=K*T2  (2)

where:

Nr1 is forward rotor speed;

Nr2 is aft rotor speed;

Npt is the power turbine speed;

T1 is forward rotor torque;

T2 is aft rotor torque;

C is a constant from the gear system; and

K is a constant from the gear system.

In isochronous speed governing fixed speed control, rotor speed ismeasured and is desired to be held constant. The rotor control module 58may increase or decrease blade angle to absorb more or less power asprovided by the gas turbine engine to maintain constant rotor speed. Theincrease or decrease signal is noted as BetaDOT for “rate of change ofBeta.” For stable governing in the fixed speed control mode it isdesirable to measure rotor blade angle. For ground handling operationssuch as taxi and reverse operation, it is desirable to operate thesystem in the Beta control mode. Beta feedback is required for thatpurpose.

Due to the proximity of the forward rotor 24 to the stationary structureof the gas turbine engine 22, conventional technology may be used toprovide speed (Nr1) and Beta feedback to the rotor control module 58.The aft rotor 26 is relatively remote from the stationary structure ofthe gas turbine engine 22 and signals from the aft rotor 26 must passthrough at least two rotating interfaces as well as the gear system 38.

To control the blade pitch of the counter-rotating un-ducted rotors 24,26, two feedback loops are communicated to the rotor control module 58.A Blade Angle feedback signal (Beta1 Feedback) and a commanded rate ofchange of the blade angle signal (Beta1DOT commanded) communicate withthe forward rotor 24. A Blade Angle feedback signal (Beta2 Feedback) anda commanded rate of change of the blade angle signal (Beta2DOTcommanded) communicate with the aft rotor 26. In control parlance,commanding a rate such as changing a rotor blade angle at 3 degrees persecond in the positive direction, is a rate command and is usually givena DOT for the first derivative—so BetaDOT would be rate of change ofBeta.

Rotor blade angle actual and rotor speed Nr1 may be measured directlythrough, for example, dual magnetic sensors attached to stationarystructure adjacent to the forward rotor 24 for communication to therotor control module 58.

The rotational speed of the aft rotor 26 is calculated (Nr2 derived)from the mathematical functions above as follows—It is common andnecessary for manufactures of gas turbine engines to measure the speedof the power turbine 36 to provide back-up protection against accidentaloverspeeding and to provide underspeed governing during Beta Modeoperation of the rotor system. This signal is normally provided to theengine control 82 for that function and therefore is available to therotor control 58. Nr2 may then be calculated from the formulaNr2=C*Npt−Nr1. That is, in this embodiment, the aft rotor speed isderived—not measured. With Beta1, Beta1DOT, Beta2, Beta2DOT, and therotational speed of the aft rotor 26 (Nr2) speed governing is readilyachieved to control blade angle and prevent the engine 22 from overspeeding and maintain power within the desired limits throughout variousregimes. But this requires the Beta2 and Beta2DOT signals becommunicated through a multiple of rotational interfaces between the aftrotor 26 and the rotor control module 58 which may be somewhatcomplicated.

In this configuration, the aft rotor 26 is fully usable for ground andreverse operation under Beta Control in the same manner of the forwardrotor 24 (FIGS. 6 and 7). That is, a linear relationship is providedbetween Beta1 Actual and Beta1 Commanded as well as between Beta2 Actualand Beta2 Commanded. This linear relationship permits a commandednegative pitch or a commanded positive pitch and the rotors 24, 26 willgo to that commanded pitch. There are thus no Beta restrictions onground operation or reverse operation.

With reference to FIG. 8, to measure the Blade Angle feedback signal(Beta2 Feedback) from the aft rotor 26, a feedback assembly 90 isconnected to the aft pitch change actuator 56. The feedback assembly 90generally includes a feedback shaft 92, a LVDT 94, a sliding joint 96and bearings 98A, 98B. The feedback shaft 92 is held rotationallystationary upon bearings 98A, 98B to provide a rotationally stationarypath from the aft pitch change actuator 56 to the LVDT 94. Bearings 98Apermits rotation between the feedback shaft 92 and the forward rotor 24while bearing 98B permits rotation between the feedback shaft 92 and theaft rotor 26.

The feedback assembly 90 directly monitors axial position of the aftpitch change actuator 56. Such feedback may be required only belowFlight Idle. The feedback shaft 92 includes an axial stop 92S such thatthe feedback shaft 92 is axially restrained above Flight Idle. Furtheraxial movement of the aft pitch change actuator 56 above flight idle isabsorbed by the sliding joint 96 such as a spring which compresses aboveflight idle to minimize the stroke applied to the LVDT 94. The strokelimitation increases the fidelity of measurement required of the LVDT94. It should be understood that various other measurement systems mayalternatively or additionally be provided.

With Reference to FIG. 9, another non-limiting embodiment of a controlsystem 100 with inputs and outputs to control the counter-rotatingun-ducted rotors 24, 26 of the CROR 20. In this non-limiting embodiment,the rotational speed of the aft rotor 26 is calculated (Nr2 derived)from the mathematical functions as discussed above and the Beta2feedback signal is eliminated.

To command the aft rotor 26 for either increase pitch or decrease pitch,enough information is available because of the mathematicalrelationships of the gear system 38. That is, the power turbine outputspeed Npt is measured and the rotor speed Nr1 of the forward rotor ismeasured, then with the mathematical relationships, the rotor speed Nr2of the aft rotor 26 is calculated. Control of rotor speed in the speedgoverning mode is accomplished in the same manner as described above forsystem FIG. 5 with the exception that the dynamic gains used fordetermining Beta2 Dot must be designed to be compatible with the leaststable operating condition.

Since the control system 100 does not receive the Beta2 feedback signal,the aft rotor 26 is prevented from entering Reverse pitch by theintroduction of a stroke limit or hard stop in the actuation linkage.That is, since Beta Control for the aft rotor 26 is eliminated, the aftrotor 26 is prevented from entering a pitch below the low pitch stop(FIG. 3). Beta Control operations (below the low pitch stop) such asreversing, and other such ground operations are performed by the forwardrotor 24 alone as the aft rotor 26 is held fixed at the low pitch stopwhenever the forward rotor 24 is commanded to a lower pitch than the lowpitch stop.

Any time the control system 100 is in beta control mode, the aft rotor26 is on the low pitch stop and the forward rotor 24 will alone moveinto the low blade angles and into reverse. In this configuration, theaft rotor 26 is not used for ground and reverse operation under BetaControl as is the forward rotor 24 (FIGS. 10 and 11). That is, a linearrelationship is provided between Beta1 Actual and Beta1 commandedwhereas the linear relationship between Beta2 Actual and Beta2 Commandedis cut off at the low pitch stop.

In one non-limiting embodiment, the aft rotor 26 is physically limitedto the low pitch stop by physically limiting the stroke of the aft pitchchange actuator 56. That is, during ground operation the aft rotor 26may be positioned against the low pitch stop (often referred to as theFlight Idle Stop) and forward and reverse thrust is controlled by acombination of engine power and beta control of the forward rotor 24.

As an alternate control approach, the aft rotor 26 may be positionedagainst a feather hard stop (FIG. 3) since use of the mechanical lowpitch stop on the aft rotor 26 requires ground thrust control basedsolely on blade pitch changes of the forward rotor 24. With the aftrotor 26 at a flight idle blade angle, a relatively greater amount ofreverse thrust is required from the forward rotor 24 due to the forwardthrust generated by the aft rotor 26 when reverse power is applied asthe aft rotor 26 is limited to the low pitch stop. To maximize netreverse thrust, the aft rotor 26 may alternatively be commanded to theblade angle (feathered) to minimize the forward thrust from the aftrotor 26. This may be particularly advantageous for reverse thrustapplication after touchdown to minimize aircraft stopping distance.

While minimizing the forward thrust generated by the aft rotor 26 isadvantageous from an aircraft stopping performance perspective aftertouchdown, minimization of the rotational speed variation as well asminimization of the average rotation speed of the aft rotor 26 inreverse thrust operation may also be advantageous. Minimizing the zonesof potential speed operation in the aft rotor 26 may be critical toensuring that the aft rotor is not operated continuously at a speed thatmay excite any rotor blade natural frequencies. This avoidance willminimize the potential for fatigue damage accumulation in the rotorassemblies.

In the unlikely event of a loss of propulsion system hydraulic pressure,counterweights 102 (FIG. 12) are mounted to the base of each of therotor blades 24B, 26B to provide the force output necessary to drive therotor blades 24B, 26B towards increased pitch so as to provide a safefailure mode through elimination of any potential engine overspeedcondition.

An independent electronic overspeed and low pitch stop protectionsystem, such as that disclosed in U.S. Pat. No. 6,422,816 B1, entitled“VARIABLE PITCH PROPELLER CONTROL SYSTEM,” which is assigned to theassignee of the instant disclosure and which is hereby incorporatedherein in its entirety, provides protection in the event of controlsystem failure which may otherwise result in an increase in rotor RPMoutside of established limits or a commanded blade angle belowestablished limits. In the event of a complete loss of electrical powerto the rotor control system, the rotors 24, 26 are driven hydraulicallytowards high pitch (feather) to avoid overspeeds as well as in-flightlow pitch stop violations. This may be accomplished by the applicationof an electrical null bias in the electro-hydraulic servo-valve suchthat with no electrical input to the valve, the porting is such that theactuators 54, 56 are always hydraulically driven in the increase pitchdirection.

Another non-limiting embodiment limits movement of the aft rotor 26 tothe low pitch stop through a pitchlock such as that disclosed in UnitedStates Patent Application No. 2007/0212220A1, entitled “CONTROLLEDPROPELLER PITCH LOCK ACTUATION SYSTEM,” which is assigned to theassignee of the instant disclosure and which is hereby incorporatedherein in its entirety. In this non-limiting embodiment multiplepitchlocks may be symmetrically around the forward and aft rotoractuators which lock-up and prevent unwanted travel in the decreasepitch direction upon loss of hydraulic power. This permits reduction orelimination of the counterweights thus resulting in reduced systemweight.

With reference to FIG. 13, another non-limiting embodiment of a controlsystem 110 with inputs and outputs to control the counter-rotatingun-ducted rotors 24, 26 of the CROR 20 is schematically illustrated. Inthis non-limiting embodiment, pitch change systems 50, 52 aremechanically linked such as through a ball screw. The blade angle of theforward rotor 24 (Beta1) is commanded and the blade angle of the aftrotor 26 (Beta2) follows such that there is a mechanical functionalrelationship B2=f(B1). The relationship is predetermined as described inUnited States Patent Application No. 2010/0310369 A1 entitled “PITCHCHANGE ACTUATION SYSTEM FOR A COUNTER-ROTATING PROPELLER,” which isassigned to the assignee of the instant disclosure and which is herebyincorporated herein in its entirety. This configuration eliminates theneed for Nr2 and Beta2 feedback signals as well as the command signalBeta2DOT from the aft rotor 26. This minimizes complexity.

In this configuration, the aft rotor 26 is used for ground and reverseoperation under Beta Control (FIGS. 14 and 15). Although a linearrelationship is provided between Beta1 Actual and Beta1 commanded, anon-linear relationship between Beta2 Actual and Beta1 Commanded resultsfrom the control of the aft rotor 26 through the forward rotor 24.

The non-linear relationship between Beta2 Actual and Beta2 Commanded maybe through a scheduling function provided, for example, mechanicallywith a cam or other mechanical linkage such that the power absorption ofthe rotors 24, 26 are commanded with one signal. So Beta1 is commandedsuch that Beta1 actual is linear while Beta2 is a nonlinear function.

Although the aft rotor 26 may be somewhat less accurately controlledthan the forward rotor 24, the acoustics of the CROR 20 may actually beimproved as noise generation is reduced because the speeds of the rotors24, 26 are slightly different. As the rotors 24, 26 are in series in theairflow, the power absorption of each rotor is different such that whenoperated in unison the function advantageously reduces noise generation.In other words, one rotor 24, 26 will be running at the commanded speedwhile the other rotor 26, 24 will be somewhere close but different suchthat end result is a relatively quieter CROR 20.

It should be understood that like reference numerals identifycorresponding or similar elements throughout the several drawings. Itshould also be understood that although a particular componentarrangement is disclosed in the illustrated embodiment, otherarrangements will benefit herefrom.

Although particular step sequences are shown, described, and claimed, itshould be understood that steps may be performed in any order, separatedor combined unless otherwise indicated and will still benefit from thepresent disclosure.

The foregoing description is exemplary rather than defined by thelimitations within. Various non-limiting embodiments are disclosedherein, however, one of ordinary skill in the art would recognize thatvarious modifications and variations in light of the above teachingswill fall within the scope of the appended claims. It is therefore to beunderstood that within the scope of the appended claims, the disclosuremay be practiced other than as specifically described. For that reasonthe appended claims should be studied to determine true scope andcontent.

1. A method of controlling a Counter-Rotating Open-Rotor (CROR)comprising: measuring a speed of a first rotor to obtain a measuredspeed; deriving a speed of a second rotor to obtain a desired speed; andcontrolling a pitch of the first rotor and a pitch of the second rotorin response to the measured speed of the first rotor and the desiredspeed of the second rotor.
 2. The method as recited in claim 1, whereinthe first rotor is a forward rotor and the second rotor is an aft rotoralong a common axis of rotation.
 3. The method as recited in claim 1,wherein a gear system which drives the first rotor and the second rotoraxially is located between the first rotor and the second rotor.
 4. Themethod as recited in claim 1, wherein deriving the speed of the secondrotor occurs in all control modes.
 5. The method as recited in claim 1,further comprising measuring a Blade Angle feedback (Beta2 Feedback) ofthe second rotor.
 6. The method as recited in claim 1, furthercomprising measuring a Blade Angle feedback (Beta2 Feedback) of thesecond rotor only below Flight Idle.
 7. The method as recited in claim1, further comprising preventing the second rotor from entering acoefficient of power (CP) bucket when under Beta Control.
 8. The methodas recited in claim 1, further comprising restricting the second rotorto a low pitch stop when under Beta Control where rotor pitch isdirectly commanded.
 9. The method as recited in claim 1, furthercomprising mechanically holding the second rotor at a low pitch stopwhen under Beta Control where rotor pitch is directly commanded.
 10. Themethod as recited in claim 1, further comprising at least one pitch lockwhich prevents travel in a low pitch direction below a predeterminedpoint.
 11. The method as recited in claim 1, further comprising holdingthe second rotor at a feather stop when the first rotor is in Reversewhen under Beta Control where rotor pitch is directly commanded.
 12. Amethod of controlling a Counter-Rotating Open-Rotor (CROR) comprising:entering Beta Control where rotor pitch is directly commanded; andpreventing one rotor of a first rotor and a second rotor from entering acoefficient of power (CP) bucket.
 13. The method as recited in claim 12,further comprising restricting the one rotor of the first rotor and thesecond rotor to a low pitch stop.
 14. The method as recited in claim 12,further comprising restricting the one rotor of the first rotor and thesecond rotor to a feather stop when the other of the first rotor and thesecond rotor is in Reverse.
 15. A Counter-Rotating Open-Rotor (CROR)comprising: a first rotor; a second rotor; and a rotor control moduleoperable to derive a speed of one of said first rotor and said secondrotor and measure a speed of the other one of said first rotor and saidsecond rotor.
 16. The CROR as recited in claim 15 wherein said one ofsaid first rotor and the second rotor is an aft rotor in a CROR pusherconfiguration.
 17. The CROR as recited in claim 15 further comprising agear system which drives said first rotor and said second rotor, saidgear system axially between said first rotor and said second rotor. 18.The CROR as recited in claim 15 wherein said rotor control modulereceives a Blade Angle feedback (Beta2 Feedback) of said second rotor.19. The CROR as recited in claim 15 wherein said second rotor isprevented from entering a coefficient of power (CP) bucket during BetaControl where rotor pitch is directly commanded.
 20. The CROR as recitedin claim 15 wherein a pitch of said second rotor is restricted to a LowPitch Stop during Beta Control where rotor pitch is directly commanded.